I�m not familiar with the battery fixing method on a Peugot, but generally the method used is common to most cars, in that a plate is bolted to the battery shelf. The plate is so shaped to hold the battery in position and clamp over the small lip at the battery base.
I had the same problem on my wife�s Escort. Although not deliberate, I sheared the head off both bolts, trying to get them off. Fortunately the battery shelf was provided with additional threaded holes at the correct spacing. All I needed to do was purchase two bolts and reattach the plate, moved along an inch or so.
Before shearing off the bolts, I would try some oil or WD40 lubricant (leave it to soak overnight) then give it another go. If you can�t budge them � then you will have to call out a mobile mechanic. Unless you are happy to shear the bolts and if necessary, drill out and re-tap the holes.
Another idea is to jump-start the car � drive to a garage and let them deal with the problem. You may end up paying slightly over the odds for the battery � but it might save a lot of grief.